Cylindrical magnetic domains in thin anisotropic single crystal layers of ferrimagnetic materials, usually termed bubbles, are well known. Field access domain devices have been described in the article "Magnetic Bubbles" by A. H. Bobeck and H. E. D. Scovil in Scientific American, Vol. 224 page 78. In these devices, domains are generated and moved along defined propagation tracks of magnetically soft elements, and components are incorporated for domain annihilation and loop to loop transfer. The ferrimagnetic layer is characterized by a uniaxial anisotropy normal to the plane of the layer which allows the bistable existence of cylindrical domains with their axes in the easy direction of magnetization. A rotating magnetic field in the plane of the magnetic track elements causes a pole pattern in these elements, thereby establishing periodically changing field gradients which interact with the bias field and provide the driving force for their movement. In these memory devices it is also required that for read-out of the binary information stored via the circulating domains, a magnetic field sensitive detector be incorporated. The detector must be particularly sensitive to the field of the on track, moving domains.
Among the types realized and described in the literature are silicon and indium antimonide Hall-effect detectors, Faraday rotation optical detectors and magneto-resistance detectors fabricated from ferromagnetic thin films. Hall-effect detectors require deposition and fabrication control of an additional material without significant improvement of the signal output of the detector and add an additional pair of Hall-voltage sensing leads. Optical detectors are complex in that they require a light source, polarizers and a light detector. The efficiency of the nearly crossed light attenuating polarizers is very low, and the lens-slit system to focus on a diameter the size of a bubble diameter (1 - 6.mu.m) has to be rather sophisticated and costly. Although this type of detector is free from magnetically induced background signals, it has its own detector noise problems, is bulky and, as mentioned, is costly compared to magnetic film sensors. The magneto-resistance sensor is most practical and widely used since it can be fabricated from the same soft ferromagnetic material as the bubble propagation structure, either in a second deposition as a "thin film detector" or fabricated in one fabrication step with the propagation structure as a "thick film detector".
The magneto-resistance change .DELTA.R is due to a transition between two different magnetized states in the plane of the magnetic film. For a polycrystalline film the resistivity, as a function of an in-plane field or the radial component of a magnetic domain, is given by: (1) EQU .rho. = .rho..perp. + .DELTA..rho. (H) cos.sup.2 .theta.
where .theta. is the angle between the current and magnetization vectors of the magneto-resistance device. EQU .DELTA..rho. (H) = (.rho..parallel. - .rho..perp.) (2)
the above equation defines the resistivity change for the in-plane magnetic field in directions parallel and perpendicular to the current through the device. The magneto-resistance detector is most compatible technologically and has a reasonably large .DELTA..rho./.rho. at room temperature.
The detector is most efficient when its size is comparable to the bubble or stretched strip domain dimensions, but demagnetizing fields are very high in detectors of this small size; for example, for 80 - 20 permalloy .DELTA..rho./.rho. is approximately 5% and for films it is in the range from 1% to 2%. In small detectors .DELTA..rho./.rho. is reduced to 0.5% to 1%. The bubble radial field, switching the detector, is of the order of 20 Oe which does not completely saturate the detector, thereby reducing the output from the detector further. These factors indicate the desirability of finding a way of increasing the output from magneto-resistance detectors. The output of the detector is given by: EQU .DELTA.V = I.DELTA.R (3)
for a constant current through the detector, where .DELTA.R is due to the magnetic field change to be detected. ##EQU1## where .DELTA..rho. is given in equation (2), l is the effective detector length, w is the detector width and t is the thickness of the detector. .DELTA.V is inversely proportional to the thickness, t, since .DELTA..rho. is relatively insensitive to t above 200A. Typically, the thin film detector is fabricated from 80 - 20 (Ni - Fe) permalloy in the thickness range from 200 to 400A. Films of this thickness are difficult to fabricate reproducibly and when operated at optimum signal output (50% of burn-out current), problems arise from short detector life in some cases. The thick film permalloy detector has a thickness in the range from 2000 to 4000A, depending on the design of the permalloy track and the coupling factor between the bubble and permalloy fields. According to equation (4), the thick film detector has a smaller output for the same current and in-plane dimensions.
Most commonly, permalloy detectors are the longitudinal stretch (Chinese letter) or transverse chevron type. The Chinese letter detector is usually used as a "thin film detector" such that the sensing element does not interfere with propagation. The output of this detector is a single pulse as the domain is stretched across the sensing element in less than 1/2 a period of the rotating magnetic field. Stretching in the direction of propagation is frequency-limited by the stretching mobility of the domain. Usually three or five stretching bars are used. With increasing frequency stretching is not completed, which results ultimately in domain hang-up at the detector. To increase frequency performance a shortening of the stretcher results in a shorter detector and according to equation (4) in a smaller signal.
A typical output from a 400A thick five stretcher detector is in the range from 0.5 to 0.8 mV at 66 kHz and 4 mA detector current. The most advanced type of the chevron detector, which consists of a stretching and a destretching ramp of chevron stacks, has its center conductor interconnections of alternate chevrons to provide a long magneto-resistive sensing element. Its advantages are that there are no inherent frequency limitations, assuming the ramp is long enough, since the stretching is transverse to the direction of propagation and that it can be fabricated together with the propagation structure in the same fabrication step. Dependent on the design of the chevron interconnections and their positions on the chevrons, one observes wide and multiple output peaks which can interfere with the read-out of adjacent bubbles and make it difficult to multiplex signals from several storage devices. The signal output from 3000A thick chevron detector with 18 chevrons gives a peak signal of 1 mV for 4 mA where the output is up to 2.5 mA at 100 kHz. Here the signal loss due to a "thicker film" is compensated by design of a longer stretch and sensing element.
The system environment in which the detector is located also affects the detector output. The magnetization in the sensor of the detector is switched by the field of the domain and also by the rotating field and the influence of neighboring permalloy elements. A detector produces a coherent magneto-resistance background signal due to the in-plane, rotating field, the signal being at twice the frequency of the rotating magnetic field. This so-called 2f-signal at a rotating field of 30 Oe is 2mV for the thin film Chinese letter detector and 4.2 mV for the chevron detector at 4 mA. Additional background signal is generated in the voltage lead-out loop of the detector due to inductive pick-up by the area extended normal to the rotating field or by components of the fringe field at the edge of the rotating field coil. This inductive pick-up signal is in the range from 2 to 10 mV. The signal is somewhat dependent on the design and is largely minimized by close spacing of the detector leads on the memory board leading out of the field module. In an actual system environment, domain detector signals have to be extracted out of larger background signals. The large common-mode signals are minimized by differential sensing of signals from two similar detectors to buck out the 2f signal, and similar inductive loops are used to cancel the d.phi./dt pick-up. The inductive voltage pick-up is given by: EQU E = 2.pi.f BA (5)
where
f = drive field frequency
B = magnetic induction
A = area extended normal to the magnetic flux of the drive field
Significant here is that the induced voltage is proportional to the frequency and further illustrates the requirement of careful design at high frequencies. At a drive field of 30 Oe and a frequency of 100 kHz the peak pick-up voltage per unit area is ##EQU2##